The present invention relates generally to semiconductor devices, and more specifically to a trench-gated semiconductor device having trench walls formed by selective epitaxial growth.
The term xe2x80x9ctrench MOSFET devicexe2x80x9d is used in this specification to refer to a device with multiple sources separated by gated trenches and sharing a common drain.
A typical prior-art trench MOSFET device is made up of an assembly of sources, each separated by gate regions constructed inside a trench with gate dielectric located on all its sides. The trench is filled with polysilicon used as a gate electrode. Source connection is achieved using thick top metal, through the gate-source dielectric opening, by direct contact with silicon source and body regions. The backside (bottom) of the N+substrate is used as a drain. A typical trench MOSFET device consists of an array of sources and gates arranged in various cellular or stripe layouts currently used by the industry. Such devices are typically made with a substrate and multiple deposition, ion implantation, masking and removal steps. It would be desirable to reduce the number of steps or simplify the overall process of fabrication.
The formation of the N+source and P+body regions of trench MOSFET devices is usually accomplished by implantation and thermal activation and diffusion. It would be advantageous to eliminate the separate precise steps necessary for such formation processes, and accomplish the same or better results using selective epitaxial growth and etch back. A thick oxide is grown or deposited on an N-type epitaxial layer that is on top of a N-type silicon substrate to define the height and width of the intended trench. The body region is partially formed by selective epitaxial growth (SEG) and etch back. Source regions are then formed by SEG. The body is completed by SEG and etch back, followed by trench oxide removal. Without added fabrication steps, the etch back of the body region gives the device a recessed body, which offers opportunities both to improve device UIS (unclamped inductive switching) capability and to increase contact area to reduce contact resistance. A wet etch of the trench sidewalls avoids residual effects from plasma etching of the silicon, such as damage from high-energy plasma elements and unwanted etch-rate variation. Gate oxide is then grown or deposited in the gate trench. After gate oxidation, the P-type epitaxial layer will laterally and vertically diffuse around the trench comers, offering alleviation of trench-corner-related reliability problems. Gate material is then deposited to fill the trench. Excess oxide is removed from the surface. An interlevel dielectric material is deposited on top of all the diffusions and trench gate region. A contact etch then exposes all N+source and P+body planar contact area, while leaving dielectric intact inside the trench. As in prior-art devices, source/body contact metal is deposited on the top surface and drain contact metal is deposited on the backside of the wafer.